Ion implanters are known for use in treating workpieces such as silicon wafers. When such a wafer is bombarded with an ion beam, the silicon wafer is selectively doped with the ion impurity to create a semiconductor material from the original silicon material of the wafer. It is well known to create complex integrated circuits using such wafers. Circuit components on a small scale are created through use of masking techniques that selectively limit the exposure of the silicon to bombardment with the ions coming from an ion source.
Techniques for use with an ion implanter that use other than silicon substrates are becoming more common. Silicon-on-insulator technology involves use of an insulating base that supports a thin layer of silicon. Other processes use so called strained silicon. In this use, silicon is grown on top of a layer of silicon germanium so that the atoms in the silicon layer align with those in the slightly larger crystalline lattice of the SiGe. Other processes bond together wafers that are processed separately to produce a combined function circuit that operates as if it were processed on a single substrate.
Ion implanters generally fall into different categories that depend on their intended use. One class implanter has a support that supports multiple wafers which are moved through a suitably shaped ion beam. Other implanters treat or process one wafer at a time. In these so called serial implanters, the single wafer is mounted to a support that translates back and forth through a thin ribbon shaped beam coming from an ion source. Both type implanters include a source of ions which typically ionize a source material such as Boron in either a gaseous or solid state and selectively accelerate and filter the resultant ion species to form a ribbon beam having a controlled dose and energy.
In either type implanter, the beam is continuous and strikes all portions of the workpiece (typically a generally circular wafer) without regard to the structure of the workpiece. When used to create semiconductor devices on a silicon substrate, this means that the ion beam strikes or impinges on a photoresist layer that has been placed onto the wafer during previous processing at a different process station. The wafer (and selectively applied resist) is automatically moved into and out of an ion process chamber by use of one or more robots that automate the transfer into and out of the ion implanter.
Certain prior art patents owned by the assignee of the present invention have been concerned with contamination within an ion implantation chamber. These know patents include U.S. Pat. No. 5,656,092 to Blake et al, U.S. Pat. No. 5,633,506 to Blake, U.S. Pat. No. 5,554,854 to Blake, U.S. Pat. No. 5,998,798 to Halling et al, U.S. Pat. No. 6,600,163 to Halling, and U.S. Pat. No. 6,657,209 to Halling. The subject matter of these prior art patents are incorporated herein in their entirety.
In device and integrated circuits fabrication use of patterns on the wafer is necessary to achieve selective doping. It is not unusual that process steps such as ion implantation are performed into the wafers with patterned photoresist left on their surface. The photoresist needs to be thick enough to ensure that no significant fraction of the ions reach the wafer. An illustration of such a prior art implantation process is depicted in FIG. 7. In that figure a layer of resist R covers a substrate S in selected regions and exposes the substrate to ion implantation treatment in other regions.
During the implantation process the energetic ions impinging upon the photoresist layer break hydrocarbon bonds of the resist along their path till they come to rest. Damage of the photoresist by the incoming ions result in significant outgassing. RGA (residual gas analysis) has shown that a spectrum of the mass of the volatile components emitted from the resist covers the range between mass 2 (H2) up to mass 28 (CO) or even 44 (CO2). Numerous hydrocarbon molecules are evolving in the outgassing spectrum including CH2, CH4, C2H2. The partial pressure of these constituents is significant at early stage of implant process. See T. N. Horsky “Photoresist outgassing in high energy and high current ion implantation” Ion Implantation Technology Proceedings, ISBN 0-7803-4538-x, IEEE 1999, pp 654–657.
Observation of these species in the RGA spectrum was found to be non-specific to a particular formulae of the applied photoresist, thus it may be assumed to be applicable to a broad range of polymers used for patterning the workpieces.
In general, H2 is assumed to be a major constituent of outgassing which is particularly true for implantation at high energy, and also at later stages of the implantation processes. At lower energy ranges of impinging ions and also for early stages of the implantation processes the CO/N2 peak (mass 28) has been shown to be a dominant gas load followed by H2O and then H2 (only 10% of the total gas load at this stage). This demonstrates that outgassing composition can change significantly over the wide energy range used in contemporary processes. See F. Sinclair, M. Eacobacci, Jr. “Forces of change keep reshaping vacuum setup in iom plant”, Solid State Technology, April 2004, p.27, www.solid-state.com.
Since process conditions of semiconductors extend over a broad range of energy, species and materials (new materials are being introduced) consideration needs to be given to these other chemicals released from the polymer, especially since they may dominate the composition of the gas at early stages of outgassing. In contemporary ion implanters cryogenic pumps are being used to ensure desired vacuum level in the process chamber. These pumps are particularly efficient in pumping hydrogen and dealing with gases such as Xenon (used in PEF for charge control purposes). However, increased H2 pumping speed in improved design of the cryopumps does not solve the problem of other gases. At present there is no specific approach in the pumping scheme applied to evacuate hydrocarbons evolving from the damaged photoresist.
These volatile species reach the process chamber components and walls and form a coating which grows in thickness with time of processing photoresist-covered wafers in the implanter. Since such films are insulating, they may adversely affect functionality of components if they cover conductive surfaces. If the film grows in thickness beyond some critical level internal stress as well as incompatibility of thermal properties between the film and underlying materials may lead to cracking and delamination of the film and generate particle problems within the implantation chamber.
Another adverse effect of photoresist outgassing and poor vacuum control in the process chamber pertains to the interaction of the incident ions with the gases (released from the damaged photoresist) that may lead to the change of their charge state. This process can contribute to the errors in measurement of beam current and therefore to the dose errors (because the neutralized ions maintain their energy they are implanted if they reach the wafer). In addition it may also adversely affect dose uniformity. These interactions and the problems they create are addressed by the below described exemplary embodiments of the invention.